1. Field of Invention
The present invention relates to a nanoparticle coated material capable of being used in a magnetic recording medium MRAM and the like and a method of producing the nanoparticle coated material.
2. Description of the Related Art
It is necessary to decrease a particle size of a magnetic material for use in a magnetic recording medium in order to increase a magnetic recording density. For example, in the magnetic recording medium which has widely been used as video tape, computer tape, a disk and the like, in a case in which mass of a ferromagnetic material is same, a level of noise is decreases, as the particle size becomes smaller.
A CuAu type or Cu3Au type hard magnetic ordered alloy has high crystalline magnetic anisotropy due to strain generated at the time of being ordered and exhibits a hard magnetic property even when a particle size thereof is allowed to be small (i.e., even in a state of a so-called nanoparticle), thereby being a promising material for enhancing the magnetic recording density.
When a metal nanoparticle capable of forming the CuAu type or Cu3Au type hard magnetic ordered alloy is formed, a structure thereof becomes a face-centered cubic system. The face-centered cubic system ordinarily exhibits a soft magnetic property or a paramagnetic property. A material which is soft magnetic or paramagnetic is not appropriate for an application of a recording medium.
Accordingly, in order to obtain a hard magnetic ordered alloy having a coercive force of 95.5 KA/m or more, it has been necessary to perform a heat treatment on the material at a temperature of 500° C. or more.
However, it was difficult to disperse particles, having a diameter of from 1 nm to 100 nm, which were subjected to a heat treatment on a support. Further, in a case in which an organic support comprising any one member selected from the group consisting of polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins; cellulose triacetate; polycarbonate; polyamides (inclusive of aliphatic polyamides and aromatic polyamides such as aramids); polyimide; polyamideimide; polysulfone; polybenzoxazole and the like was used, there were problems with thermal resistance of the support when a magnetic recording medium in which metal nanoparticles were applied on the support, was subjected to a heat treatment while, in a case in which an inorganic support such as glass, alumina, Si or SiO2 was used, there was also a problem of strain or the like being generated when the inorganic support was subjected to a thermal treatment. Thus, such supports as described above have not been put to practical use.
Moreover, immediately after the nanoparticles capable of forming the CuAu type or Cu3Au type hard magnetic ordered alloy were synthesized by a liquid phase method or a gas phase method, the nanoparticles were and soft magnetic or paramagnetic in a disordered phase. The nanoparticles in such state can not be used for the magnetic recording medium. In order to obtain an ordered alloy phase, it is necessary to perform an annealing treatment (thermal treatment) on the nanoparticles at about 500° to change the disordered phase of the nanoparticles into an ordered phase.
However, in a case of a glass substrate or the like in which a support comprises a material containing a large quantity of an element which causes deterioration of magnetic characteristics of Si or the like, when an annealing treatment is performed on the substrate at such a high temperature as described above, the above-described element is diffused from the support in a magnetic layer (nanoparticle layer), thereby interfering with phase transformation of the alloy constituting the nanoparticles; hence, there has been a problem of deterioration of the magnetic characteristics (particularly, coercive force Hc) of the magnetic recording medium.
Further, since a temperature of transformation from the disordered phase to the ordered phase is increased, heat strain on the support and other problems occur; such cases are not desirable industrially.
Accordingly, it is an object of the present invention to provide a method of producing a nanoparticle coated material in which a nanoparticle alloy phase can efficiently be transformed into a CuAu type or Cu3Au type hard magnetic ordered alloy phase without changing quality, shape or the like of a support regardless of whether the support is an organic substance or an inorganic substance.
It is another object of the present invention to provide a magnetic recording medium which is a nanoparticle coated material that can prevent an element which deteriorates magnetic characteristics from being diffused from a support, does not increase a transformation temperature of an alloy constituting nanoparticles and shows a hard magnetic property.